IgA is the primary immunoglobulin of the mucosal immune system and is responsible for protecting the host from invasion by pathogenic microorganisms that enter through the mucosae. In contrast to mice and humans which have at most two IgA isotopes, rabbits have 13 IgA isotopes and these are differentially expressed in various mucosal tissues. Our goal is to elucidate the molecular basis for isotype switching, especially how the differential expression of the IgA isotypes in different tissues is regulated. As a prerequisite to IgA switch recombination, sterile Ialpha-Calpha transcripts are induced by the Ialpha promoter regions in response to stimulation by TGF-beta-induced transcription factors. Our primary focus is to compare the Ialpha promoter regions of the 13 different IgA heavy chain genes, in vitro, by electrophoretic mobility gel shift assays and by luciferase reporter gene assays. We have found that some Ialpha promoter regions have defective transcription binding sites and we plan to compare the transcription factor binding sites of the multiple Ialpha promoter regions to determine how these contribute to the differential expression of Calpha genes in the mucosae. We will also determine to what extent the differential expression of the Calpha genes is determined by differential responses to the 3' alpha enhancer region. Finally, we will determine whether the differential expression of IgA in the gut mucosae results from local isotype switch recombination or differential lymphocyte homing. These experiments are important because they will help elucidate the regulatory steps in isotype switch recombination, including the role that environmental factors in various regions of the mucosae play in this process. This knowledge will be valuable for developing more effective mucosal vaccines.